1. Field of the Invention
The present invention relates to a surface acoustic wave element that is operated in a high-frequency range including several hundreds of MHz to GHz and that has a high resistance to effects of the environment. More particularly the invention relates to the structure of the surface acoustic wave element.
2. Related Background Information
A surface acoustic wave element is an electro-mechanical conversion element utilizing a surface wave propagating on the surface of an elastic body and has a basic structure shown in FIG. 14. In a surface acoustic wave element 40 shown in FIG. 14, the piezoelectric phenomenon of a piezoelectric body 44 is used in exciting a surface acoustic wave. When an electrical signal is applied to one comb-like electrode 43 formed on the piezoelectric body 44, the piezoelectric body 44 is stressed, and this stress becomes a surface acoustic wave. The surface acoustic wave propagates on the piezoelectric body 44 and is extracted as an electrical signal at the other comb-like electrode 43'. The frequency characteristics of the surface acoustic wave element are given as band passing characteristics having, as a center frequency, a frequency f.sub.0 proportional to a ratio V/.lambda..sub.0, which is a ratio of a propagation speed V of the surface acoustic wave to a pitch period .lambda..sub.0 of the segments of each comb-like electrode.
A surface acoustic wave element requires a smaller number of parts and can be made compact. In addition, signals can be easily input and output on a surface acoustic wave propagation path. This element can be used in a variety of applications, for example functioning as a filter, a delay line, an oscillator, a resonator, a convolver, or a correlator. In particular, a surface acoustic wave filter has been used as an IF television filter. Such surface acoustic wave filters have also been used in VTRs and as filters in various communication equipment such as mobile and portable telephones.
A typical conventional surface acoustic filter is an element having a structure in which comb-like electrodes are formed on a crystalline piezoelectric material such as LiNbO.sub.3 or LiTaO.sub.3. Another surface acoustic wave element has a piezoelectric thin film made of ZnO or the like sputtered on a substrate such as a glass substrate.
It is, however, difficult to manufacture an element operated in a high-frequency (Ghz band) range using the conventional element structure described above. An element obtained by forming comb-like electrodes on a monocrystalline piezoelectric body cannot have a high center frequency of 1 Ghz or more because it has a low surface acoustic wave propagation speed V.
As indicated by the above equation, in order to cause a surface acoustic wave element to have band passing characteristics with a higher center frequency, the pitch period .lambda..sub.0 must be reduced, or the surface acoustic wave propagation speed V must be increased.
Increasing the center frequency by decreasing the electrode pitch period .lambda..sub.0 of the comb-like electrode is limited by micropatterning techniques such as photolithography.
For this reason, various techniques for increasing the propagation speed V of the surface acoustic wave have been examined. Such known techniques are disclosed in Japanese Patent Laid-Open Nos. 54-38874, 64-62911, 1-103310, and 3-198412.
A surface acoustic wave element used as a filter in the fields of mobile communications is required to be used at a higher frequency of 1.5 to 3 Ghz in accordance with the assignment of frequencies. To realize such a filter, an element structure capable of highly efficiently performing electro-mechanical conversion and having a higher surface acoustic wave propagation speed V is required.
It has been considered to use a surface acoustic wave element as a high-frequency filter particularly in a portable telephone or the like. As is well known, components incorporated in this telephone are adversely affected by humidity and impurities because the portable telephone is used in various environments. Therefore, the surface acoustic wave element used in the portable telephone must always maintain stable characteristics in various application environments.
The present invention has been made in consideration of these conventional technical problems, and has as its object to provide a surface acoustic wave element which can be used in a high-frequency range and can maintain stable performance in various external environments, i.e. is highly resistant to environmental effects.